1. Field of the Invention
The present invention relates to an apparatus for detecting focusing and tracking errors, which is incorporated in an optical filing apparatus for recording, reading, or retrieving information by means of light beams, and more particularly, to an apparatus for detecting and correcting focusing and tracking errors, in which light beams are converged on an information recording medium, e.g., an optical disk, in order to record information on the optical disk or read the recorded information therefrom.
2. Description of the Related Art
Optical filing apparatuses have been developed recently that which can record image information, such as document data, retrieve the information as required, and reproduce the information as a hard or soft copy, e.g., optical disk apparatuses for recording or reproducing image information. In these optical disk apparatuses, a convergent light beam is applied to a disk-shaped recording medium or optical disk, whereby the information is recorded or reproduced. In recording the information on the optical disk, the light beam is applied to change the state of a region on a recording surface of the disk. As a result, the information is recorded in the form of pits on those tracks of the optical disk with a plurality of concentric grooves thereon. In reproducing the information, a light beam with a constant luminous intensity is applied to the recording surface, and its intensity is modulated by the pits. The intensity-modulated beam is reflected by the recording surface and then is detected by a detecting surface of a photodetector. The light beam detected by the photodetector is processed by means of a signal processing circuit, whereupon the information is reproduced. In recording and reproducing the information, the optical disk is rotated at a constant linear speed. More specifically, the disk is rotated slower when its outer tracks are used for the recording or reproduction, and faster when its inner tracks are used. At the same time, an optical head for directing the light beam to the optical disk is linearly moved in the radial direction, that is, at right angles to the tracks on the disk.
The optical head comprises an objective lens for converging the light beam on the tracks of the optical disk, supporting means for supporting the objective lens for movement in the direction of the optical axis, in order to allow the light beam converged by the lens to be accurately applied to a specific region, and shifting means for moving the lens. Thus, the objective lens is moved in the direction of the optical axis by a movable mechanism, which includes the supporting and shifting means, so that it is kept focused, and also, the light beam from the lens is converged on the tracks of the optical disk. In order to maintain the focused state of the objective lens, at this time, a focusing error on the recording surface is detected, and the detection result is fed back to the movable mechanism for the objective lens. Conventional methods of such focusing error detection include, for example, the wedge prism method.
In the wedge prism method, the light beam reflected by the information recording medium in a focused state is transmitted through a convergent lens, and is divided into two beams by means of a wedge prism. In doing this, the wedge prism gives a specific angle to the two light beams, so that the branch beams are diverged in different directions. These two divergent light beams are applied individually to the respective detecting surfaces of two photodetectors in predetermined positions, and are converted into electric signals. These electric signals are processed to produce a focusing control signal corresponding to a focusing error, and this focusing control signal is supplied to a voice coil for moving the objective lens. As the focusing control signal is supplied to the voice coil in this manner, the objective lens is moved so that the light beam is focused on the information recording medium.
In the focusing control described above, the light beam is divided, by the wedge prism, into two divergent beams which emerge in different directions, at the specific angle to the optical axis. Accordingly, the space between the two photodetectors for detecting the two beams substantially depends on the distances between the prism and the photodetectors in the direction of the optical axis. In order to achieve accurate focusing control, therefore, the space between the photodetectors, which depends on the mounting position of the wedge prism, must be adjusted when the apparatus is assembled.
A measure to counter this problem is proposed in U.S. patent application No. 271,827 (Nov. 16, 1988) by Sou Ishika.